* Proofs are hard. It's not clear to me that doing proofs is
generally the best use of my time. It *is* clear that there are cases
when it is. Figuring when to and when not to is just one of those
bits of life...

* Proofs are like comments -- easy to leave out.

* People write wrong proofs just like they write wrong programs.

* Proof techniques are not well-integrated. I know of no library
routines that even *claim* to have a formal specification.